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I have a 3 zone FHW system with 3 separate circulators. when circulator 1 or 2 come on, :cool:heat goes to the first baseboard in the 3rd zone, but does not go all the way through the 3rd zone. the temp in the loop decreases the further it goes in the loop

Are you saying the 3rd zone isn't heating or that it is heating when not calling for heat? If it's not heating and the circulator is running you may have air in that zone. If it is heating when not calling for heat you may have a bad flow check.

It sounds like you're having problems with all three check valves----especially the ones for zone 1 & zone 2.

If you're saying that the only heat you're getting now is thru the 1st baseboard of zone 3, no matter which zone is calling for heat, that indicates that the zone 1 and zone 2 check valves are stuck shut because of crud accumulation on the inside (a common occurance, especially if you have well water, or high calcium/mineral content in your water).

There is a little metal twist tab on the top of these flow checks that can be operated by hand to open & close the inner metal cup (that controls the water flow thru each zone)----try twisting this tab with your thumb & forefinger to see if you can get it moving.

If you can get it moving, "exercise" the twist tab/metal cup by twisting it back & forth at least 10 times until you get free, easy movement---you should be able to hear the inner metal cup opening & closing as you do the movements.

Sometimes tapping the top of the valve (gently) with the handle of a screwdiriver will help.

If the previous 2 procedures don't loosen things, GENTLY twist the metal tab with a pliers attached back & forth to see if you can get the inner cups moving again.

If the previous attempts don't work, you'll have to shut down the system drain the water out of the piping, & remove the upper half of the flow check and clean out the crud by hand with a metal pick/small screwdriver, grease the fitting & reassemble the unit.

Refill the system water & remove the air from the system by opening the bleed valves on each convector.